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Finchite (IMA2017-052), Sr(UO2)2(V2O8)·5H2O, is the first uranium mineral known to contain essential Sr. The new mineral occurs as yellow-green blades up to ~10 µm in length in surface outcrops of the calcrete-type uranium deposit at Sulfur Springs Draw, Martin County, Texas, U.S.A. Crystals of finchite were subsequently discovered underground in the Pandora mine, La Sal, San Juan County, Utah, U.S.A., as diamond-shaped golden-yellow crystals reaching up to 1 mm. The crystal structure of finchite from both localities was determined using single-crystal X-ray diffraction and is orthorhombic, Pcan, with a = 10.363(6) Å, b = 8.498(5) Å, c = 16.250(9) Å, V = 1431.0(13) Å3, Z = 4 (R1 = 0.0555) from Sulfur Springs Draw; and a = 10.3898(16), b = 8.5326(14), c = 16.3765(3) Å, V = 1451.8(4) Å3, Z = 4 (R1 = 0.0600) from the Pandora mine. Electron-probe microanalysis provided the empirical formula (Sr0.88K0.17Ca0.10Mg0.07Al0.03Fe0.02)Σ1.20(UO2)2(V2.08O8)·5H2O for crystals from Sulfur Springs Draw, and (Sr0.50Ca0.28Ba0.22K0.05)Σ0.94(U0.99O2)2(V2.01O8)·5H2O for crystals from the Pandora mine, based on 17 O atoms per formula unit. The structure of finchite contains uranyl vanadate sheets based upon the francevillite topology. Finchite is a possible immobilization species for both uranium and the dangerous radionuclide 90Sr because of the relative insolubility of uranyl vanadate minerals in water.

Understanding the crystal chemistry, materials properties, and thermodynamics of uranyl minerals and their synthetic analogs is an essential step for predicting and controlling the long-term environmental behavior of uranium. Uranyl vanadate minerals are relatively insoluble and widely disseminated within U ore deposits and mine and mill tailings. Pure uranyl vanadate mineral analogs were synthesized for investigation using high-temperature drop solution calorimetry. Calculated standard-state enthalpies of formation were found to be −4928.52 ± 13.90, −5748.81 ± 13.59, and −6402.88 ± 21.01, kJ/mol for carnotite, curienite, and francevillite, respectively. Enthalpies of formation from binary oxides for uranyl vanadate minerals exhibit a positive linear correlation as a function of the acidity of oxides. Normalized charge deficiency per anion (NCDA) is presented to relate bonding requirements of the structural units and interstitial complexes. An exponential correlation was observed between NCDA and energetic stability (enthalpy of formation from binary oxides) for the studied minerals. Additionally, NCDA and oxide acidity exhibit an exponential correlation where decreasing oxide acidity results in an exponential decrease in NCDA. The number of occurrences of uranyl vanadate mineral species are found to correlate with both enthalpy of formation from oxides and NCDA.